drm/ast: Include <linux/of.h> where necessary

Include <linux/of.h> to get of_property_read_u32() in the source
files that need it. Avoids the proxy include via <linux/i2c.h>.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-2-tzimmermann@suse.de